Membrane-assisted fluid separation processes are used to separate fluid mixtures into permeate and retentate portions. These processes may be effected within fluid separation modules that contain a plurality of hollow fiber membranes arranged in an elongated bundle encased in a single shell containment housing. The conventional fluid separation modules using hollow fiber membranes may be configured in either a shell side feed design or a bore side feed design.
Typically, fluid separation modules containing a plurality of hollow fiber membranes arranged in a bundle have potting material, for example epoxy, or other suitable material covering a portion of the external surface of each membrane within a bundle, for purposes of securing the membranes within a module. If the resin is not properly applied and leakage of feed occurs from any of the membranes, such leakage results in the contamination of permeate extracted from the hollow interior of the membrane. Similarly, if leakage-occurs for any other reason, contamination of the fluid permeate results. Accordingly, one of the disadvantages of having a single module containing hundreds to thousands of membranes is that a defect in just one membrane renders the entire module, with all of the remaining intact membranes, useless.
In order to avoid this limitation, prior art devices use a large number of modules interconnected with one another in serial or parallel fashion, in order to increase the number of hollow membranes used and thus to increase the total membrane surface area across which a given fluid mixture can be separated into its constituent permeate and retentate portions. If leakage occurs in any module, it can be replaced while minimizing the number of usable membranes discarded in the process.
These prior art devices still present two major problems. First, if there is a defect in a given membrane within a module that houses a large number of hollow fiber membranes, the entire module containing the defective membrane must be replaced, resulting in the wastage of all other usable membranes in the defective module. Moreover, in many conventional membrane modules, the housing is made of expensive material or the physical size of said housing is so large that it renders the disposal of the housing for each module along with the membranes contained therein very uneconomical. Second, whether conventional modules are arranged in series or parallel fashion, extensive plumbing is necessary in order to connect the various modules and to remove the permeate and retentate from each module. This extensive plumbing adds significantly to the cost of manufacturing and maintaining these prior art devices. The plumbing also significantly adds to the complexity and bulkiness of these devices.
In general, thermally driven fluid separation processes within conventional membrane-assisted fluid separation modules, especially those processes in which there is a large fraction of liquid feed separated as permeate by evaporation through membranes, are very energy intensive and consuming.
FIG. 1 outlines the typical flow scheme for prior art vacuum membrane distillation operating within a conventional membrane-assisted fluid separation module. In a typical prior art membrane-assisted fluid separation module 2, permeate vapours exiting the separation module are first condensed in a condenser 4 by using a cooling fluid source such as cooling water. The condensed liquid and non-condensable portions of the permeate are then separated in a gas-liquid separation vessel 6. A vacuum pump 8 is attached to the gas-liquid separation vessel to pump out non-condensable portions of the permeate and to sustain a vacuum on the permeate side.
Extensive heat is required to preheat the feed to the temperature required for optimum operation and to provide heat for vaporization for the permeate. Also cooling means (for example, cooling water, chilled water) have to be provided to remove the heat from the permeate condenser. The operation of the process according to prior art is thus very energy intensive and wasteful, as the heat supplied is mainly lost in cooling means (e.g. cooling water etc.).